Aetna is the third-biggest health insurance provider in the United States. And the Affordable Care Act, better known as Obamacare, isn't exactly helping its bottom line.

The company said it's expecting a loss "in excess of $300 million."

Aetna says it's doing a "complete evaluation" of its state exchange business. The state exchanges are marketplaces that let people buy plans that follow Obamacare policies. Until recently, Aetna was planning on expanding into New Jersey and Indiana state exchanges.

In April, United Healthcare, the largest U.S. health provider, almost quit Obamacare entirely after losing $475 million.